This invention relates generally to processes for improving selected characteristics of electrical junctions and devices incorporating the same. More particularly, it relates to improvements in processes which utilize pulsed-laser melting in the production of electrical-junction devices.
An effective technique for the production of p-n electrical junctions in semiconductor substrates comprises doping a surface layer of the substrate by ion-implantation or thermal diffusion and then annealing the doped layer with one or more laser pulses. That technique is described in detail in U.S. Pat. No. 4,181,538, issued to J. Narayan, C. W. White, and R. T. Young, on Jan. 1, 1980. Another effective technique for producing electrical junctions comprises depositing a film of dopant on a surface of a semiconductor substrate and melting the surface by means of one or more laser pulses to effect diffusion of the dopant and annealing. That technique is disclosed in U.S. Pat. No. 4,147,563, issued to R. T. Young and J. Narayan on Apr. 3, 1979. Both of the processes referred to above provide significant advantages in the production of high-efficiency solar cells and other devices. For example, they can be conducted in air and they permit the following: localization of applied heat to a thin surface layer, precise control of the emitter dopant concentration over a wide range for a fixed junction depth (with ion implanation), and nearly complete electrical activation of dopants at concentrations well above the solid-solubility limit in nearly perfect emitter regions. Unfortunately, the resulting recrystallized layer contains electrically active point defects which limit the quality of the electrical junction.
The prior art includes the process of laser-annealing a doped crystalline silicon surface by raster-scanning it with a continuous-wave (CW) laser beam. Because high-energy annealing would damage the surface, the entire substrate is heated to a moderate temperature (e.g., 350.degree. C.) throughout the CW-laser scanning operation, which may take as long as half an hour; that is, heating of the substrate permits CW-laser annealing to be accomplished with minimal surface damage due to thermal shock. However, post-annealing at, say, 700.degree. C. for more than one hour is necessary in order to remove residual defects in the near-surface region. The use of substrate heating to achieve a reduction in laser energy density for pulsed-laser annealing is disclosed in SPIE Vol. 198 (1980), Society of Photo Optical Instrumentation Engineers.